In recent years, a digital camera that stores captured images as digital data is pervading. In this digital camera, images captured through an optical lens are photo-electrically converted using a solid-state imaging device such as a CCD (Charge Coupled Device), a CMOS (Complementary Metal Oxide Semiconductor) image sensor or the like, and then recorded as digital data.
In an image signal thus generated using such imaging devices, there may often be included a noise due to a dark signal occurring in the imaging device caused by various factors in a manufacturing process or a noise occurring in a circuit board or the like. For example, among a fixed pattern noise due to the dark signal, there is a noise due to a large amount of charges occurring at a specific imaging element resulting from such as a structural defect or the like. This noise appears as a white bright spot on a display screen. Hereinafter, this noise is referred to as a bright spot noise. The bright spot noise, which in a normal use seldom occurs very largely, however, because an amount of its occurrence depends on an exposure time and temperatures, becomes remarkably noticeable under a long time of exposure or use in a high temperature environment.
Thereby, in the digital camera, usually, a processing for reducing these fixed pattern noises by correcting an image signal from the imaging device is carried out. As an example of them, a method is known in which, from an image signal which is normally captured with a shutter opened, an image signal which is captured under the same exposure condition thereof but by completely blocking the light with the shutter closed is subtracted.
However, according to the noise reduction method described above, an influence of the noise due to the dark signal cannot completely be eliminated, and there is a case where the spot portion remains conversely as a black point on a display. This problem will be described in the following. FIGS. 18(A) and 18(B) are each a diagram indicating examples of waveforms of image signals captured in the noise reduction method described above. Further, FIG. 19 is a diagram showing an example of waveforms of image signals to be outputted in the noise reduction method described above.
In FIG. 18(A), an example of waveforms of analog signals outputted from the imaging device is indicated. In this signal, on the captured image signal, there are contained bright spot noises N1 to N7 occurring due to dark signals produced by the imaging device, in addition to noise components and a direct current type offset due to noises. In the image capturing apparatus, usually, after these signals are converted to digital signals by an A/D converter, various signal processing such as a noise elimination processing or the like is applied thereto, and then the signals are stored on a recording medium or the like.
Further, FIG. 18(B) shows an example of waveforms of a signal which was captured in a condition with the shutter closed, and outputted from the imaging device. By the way, in FIG. 18(B), respective bright spot noises corresponding to those in FIG. 18(A) are labeled with the same reference numerals. In the noise reduction processing described above, capturing of the signal in this FIG. 18(B) is executed immediately after or before the capturing of the signal in FIG. 18(A) under the same conditions such as of an exposure time or the like. For example, in a case where the signal of FIG. 18(A) is captured precedently, in a state after this signal is digitalized and stored in a predetermined buffer, the signal of FIG. 18(B) is captured immediately thereafter.
Here, the signal of FIG. 18(B), because it is captured with the shutter closed, does not contain any image signal, and is comprised only of a noise component. Therefore, by subtracting the signal of FIG. 18(B) from the signal of FIG. 18(A), a signal waveform as shown in FIG. 19 from which the minute fixed pattern noise and the direct current offset are removed is obtained.
By the way, in FIGS. 18 and 19, a maximum value in a convertible range at the time of A/D conversion of these signals is indicated by a dotted line R. In FIG. 18(A), among the bright spot noises N1 to N7 due to dark signals resulting from the imaging device, especially, the bright spot noises N1, N2 and N3 have such a high level, which exceeds the range of the A/D conversion. Thereby, respective signal levels of these bright spot noises N1, N2 and N3 become all the same value at the full scale after A/D conversion, thereby failing to reflect actual signal levels thereof.
On the other hand, in the signal waveform of FIG. 18(B), there should be present the same bright spot noises N1 to N7 due to the dark signals as those existing in the FIG. 18(A), however, because the levels of these signals are contained within the A/D conversion range, they are converted to respective signal levels digitally corresponding to actual inputs thereof. From this FIG. 18(B), it is known that respective levels of the bright spot noises N1, N2 and N3 differ from each other actually.
Therefore, in a waveform which is generated by subtracting these signals, there appear arithmetic errors as indicated in FIG. 19, thereby causing a state in which respective signal levels at the portions where bright spots occurred are dropped extremely in inverse directions. These portions remain in a state like a black hole in an image produced appearing as if a dense colored pixel (hereinafter referred to as a black point).
As described hereinabove, according to the conventional noise reduction method, there is a problem that the bright spot noise due to the dark signal cannot be completely eliminated, allowing the effect of the bright spot noise to appear as black points in the image produced, thereby degrading its image quality.
The present invention has been contemplated in consideration of the problem described above, and its object is to provide an image capturing apparatus capable of eliminating the noise due to the dark signal from the solid-state imaging device.
Another object of the present invention is to provide a noise elimination method capable of eliminating the noise due to the dark signal from the solid-state imaging device.